Computer graphics systems are commonly used for displaying graphical representations of objects on a two-dimensional display. Current computer graphics systems provide highly detailed visual representations of objects and are used in a variety of applications.
A typical computer that employs a computer graphics system is shown in FIG. 1. Referring to FIG. 1, the computer 11 includes a central processing unit (CPU) 12, a system memory 14 for storing software that is executed by the CPU 12, a graphics system 16 for processing graphics data received from the CPU 12, a local interface 18 configured to electrically interconnect the foregoing elements, and a display 21 connected to the graphics system 16 via a connection 22 and configured to display the image data generated by the graphics system 16.
The graphics system 16 breaks down objects to be represented on the display 21 into graphics primitives. "Primitives" are basic components of a image data and may include points, lines, vectors, and polygons, such as triangles and quadrilaterals. Typically, hardware and/or software is implemented in the graphics system 16 in order to render, or draw, the graphics primitives that represent a view of one or more objects being represented on the display 21.
Generally, the primitives of an object to be rendered are defined by the CPU 12 in terms of primitive data. For example, when a primitive is a triangle, the CPU 12 may define the primitive in terms of, among other things, x, y, and z coordinates and color values (e.g., red, green, blue) of its vertices. Additional primitive data may be used in specific applications. Rendering hardware in a rasterizer of the graphics system ultimately interpolates the primitive data to compute the final display screen pixel values that represent each primitive, and the R, G, and B color values for each pixel.
The graphics system 16 is shown in further detail in FIG. 2. As shown in FIG. 2, the computer graphics system 16 includes one or more geometry accelerators 23 that are configured to receive vertex data from the CPU 12 and define the primitives that make up the view to be displayed. Each geometry accelerator 23 comprises a number of specialty control units 17 for processing the image data, including for example, a transform mechanism (TRANS) 24 for performing transformations on the vertex data, such as scaling or moving a vertex in space, a clip mechanism (CLIP) 26 for clipping portions of objects that extend beyond a boundary, a light mechanism (LIGHT) 28 for enhancing the image data by simulating light conditions, and a plane equation mechanism (PLANE) 32 for defining the primitives in terms of mathematical floating point plane equations. Each of the control units 17 is typically implemented via cell logic and as separate distinct state machines. The output of the geometry accelerator 23, referred to as rendering data, is used to generate final screen coordinate and color data for each pixel and each primitive. The output 33 is passed to a floating point to fixed point (FP-TO-FIXED) transformation unit 34, which converts the geometry accelerator output 33 to fixed point format 35 and which passes the value to a rasterizer 36. The rasterizer 36 produces pixel data 37, which is communicated to a frame buffer controller 38 and then to a frame buffer 42. The frame buffer 42 serves to temporarily store the pixel data prior to communication to the display. The pixel data is passed from the frame buffer 42 through a digital-to-analog converter (DAC) 44 and then to the display 21.
The operations of the geometry accelerator 23 are highly mathematical and computation intensive. One frame of a three-dimensional (3D) graphics display may include on the order of hundreds of thousands of primitives. To achieve state-of-the-art performance, the geometry accelerator 23 may be required to perform several hundred million floating point calculations per second. Furthermore, the volume of data transfer between the CPU 12 and the graphics system 16 is very large. The data for a single quadrilateral may be on the order of sixty-four words of thirty-two bits each. Additional data transmitted from the CPU 12 to the geometry accelerator 23 includes light parameters, clipping parameters, and other parameters needed to generate the graphics image for the display 21.
It is common in geometry accelerators 23 to have a stack of processing elements 52, as illustrated in FIG. 3, including but not limited to, an arithmetic logic unit (ALU) 54, a multiplier 55, a divider 56, a comparison mechanism 57, a clamping mechanism 58, etc., along with register and random access memory (RAM) work spaces 61, 62. The processor elements are typically shared by the plurality of specialty control units 17. Each control unit 17 is capable of directing the processing activities of individual processor elements 52 to accomplish specific computational tasks.
To provide processor element access to each control unit 17, adequate control line connectivity and access control should be established between the processor elements 52 and each control unit 17. One solution to providing control line connectivity is illustrated in FIG. 3 and involves multiplexing the control lines between each control unit and each processor element 52. A multiplexer (MUX) 66 of FIG. 3 serves this purpose. The MUX 66 is controlled by a MUX control mechanism 68. The MUX control mechanism 68 provides an enable signal 69 to the MUX 66 in order to control which one of the control units 17 is allowed to access the processor elements 62 at a given time. In operation, the MUX control 68 asserts an enable signal 69 pertaining to a particular control unit 17 to the MUX 66 and a go signal 72 to the particular control unit 17. In turn, the particular selected control unit 17 generates operands and a processor start signal to begin a processing operation, which are ultimately forwarded to the stack 51. The control unit 17 accesses the stack 51 and the specific desired processing element 52 via an appropriate connection 74, MUX 66, and connection 76. The control unit 17 causes the operating processing element 52 to retrieve data from the input buffer 77 (usually, a FIFO buffer) and store a result(s) in an output buffer 82 (usually, FIFO buffer). The control unit 17 can initiate any number of operations via one or more of the processing elements 52. When the control unit 17 is done with its turn, then it asserts a done signal 84 to the MUX control 68. The MUX control 68 then asserts another go signal 72 to another control unit 17, while providing an enable signal 69 corresponding to the next control unit 17.
A problem with the foregoing design is the large number of gate levels that are required to implement the MUX 66. Another problem is that the MUX 66 increases the time needed for signals to be communicated from the control unit 17 to the processing elements 52. Gate delay alone is part of this increase. Loading also contributes to the time delay, even if a tri-state MUX 66 is employed to replace the multilayered gate arrangement. Furthermore, the aforementioned problems are magnified as the number of control units 17 and the number of processing elements 52 are increased.
A heretofore unaddressed need exists in the industry for a system and method for better interfacing control units 17 with processing elements 52 in order to optimize the performance of a geometry accelerator in a computer graphics system.